The invention relates to infrared radiation imaging devices. More particularly, the invention relates to a radiation imaging device comprising a charge-transfer shift register. The shift register may be, exclusively be a silicon charge-coupled device.
U.S. Pat. No. 4,210,922 corresponding to U.K. Patent Specification No. 1,564,107) describes an infrared radiation imaging device. The device comprises a semiconductor body having a radiation-sensitive portion in which charge-carriers are generated on absorption of infrared radiation incident on the device. The device further comprises a signal-processing portion having a charge-transfer shift register. In the charge transfer shift register, the radiation-generated charge-carriers are collected and transported to an output. An electrical signal representative of the detected radiation appears at the output.
The semiconductor material of the signal-processing portion has an energy band gap which is greater than the quantum energy of the detected infrared radiation. Means are present for depleting at least the radiation-sensitive portion of the semiconductor body free charge-carriers in the absence of the radiation. The radiation-sensitive portion is of the same semiconductor material as the signal-processing portion.
The imaging devices described in U.S. Pat. No. 4,210,922 are charge-coupled devices. It is advantageous to use a common silicon body for both the CCD signal-processing portion and the radiation-sensitive portion. Both the radiation-sensitive portion and the signal-processing portion have an energy band gap which is greater than the quantum energy of the detected infrared radiation. The radiation-sensitive portion is formed by providing at least a portion of the silicon body with a concentration of at least one deep-level impurity which provides centers for trapping majority charge-carriers which can be released upon excitation by infrared radiation in a given wavelength range. Therefore, the quantum efficiency of the device is dependent on the number of majority carriers trapped at deep-level centers. The body must be cooled to low temperatures to prevent significant thermal excitation of the centers.
In order to obtain a high detectivity with this known device having deep-level centers, a very large number of centers is desired in the radiation-sensitive portion, for example up to 10.sup.16 centers per cm.sup.2. The centers may be formed by doping the body portion with, for example, indium, thallium, gallium or sulphur, or by introducing defect levels by radiation damage to the silicon lattice, for example using proton bombardment.
In practice such high concentrations of centers may be difficult to achieve in a thin layer, for example because of solid solubility limitations. Such high concentrations may also interfere with characteristics of the signal-processing portion, for example the CCD charge transfer characteristics.
With such high concentrations of centers the radiation-sensitive portion can be depleted of free charge-carriers without breakdown only if substantially all of the deep-level centers within the depletion region are full of majority charge-carriers. If the charge state of a substantial fraction of the deep-level centers is changed in the depletion regions, then breakdown would occur. This limits the integration period for the device, i.e. the period in which the maximum radiation can be incident on a particular imaging elemental portion of the device before it becomes necessary to replenish the centers with majority carriers by refreshing. In practice with a silicon device this can limit the number of centers which may change charge state to, for example, not more than 10.sup.12 per cm.sup.2 in the integration period.